This invention relates to electronic musical instruments and, more particularly, to an electronic organ system for producing a stereophonic sound image.
A stereophonic recording of a real pipe organ sounds very realistic and pleasing, but an electronic organ locks this realism both when listened to directly or when listening to a recording of its performance. As is pointed out in U.S. Pat. Nos. 2,596,258 and 3,049,040 of Donald J. Leslie, much of this lack of realism is caused by electrically mixing the outputs of the organ's oscillators or tone generators into one channel, whereas pipe organs contain many separate sound sources which are combined differently in the two ears of the listener. The lack of realism is particularly apparent when two notes which contain frequencies very close to each other are played simultaneously; for example, celeste notes which are purposely tuned a few cycles sharp, octaves which are slightly out of tune with each other, in which case harmonics of the upper note will beat with the even harmonics of the notes which are an octave lower, and notes that are a fourth or a fifth musical interval apart and contain harmonics which because of the tempered scale are slightly out of tune with each other and produce beats. Thus, if, with the string stop on, only the keys middle C and G above it are depressed, fifth interval beating would occur. If the 4' coupler is then turned on, the following beats would occur: C with G, C with C4', C with G4', G with C4' (a fourth interval beat), G with G4', and C4' with G4'. If a string celeste is added, it is seen that there will be twenty-eight separate beat combinations of the eight notes that would be sounding. In a pipe organ this produces a beautiful ensemble effect because each of the twenty-eight combinations produces a different effect on each of the listener's two ears. In order to duplicate this ensemble effect in an electronic organ, eight separate audio channels and speakers would be required, the cost of which normally would be prohibitive and the provision of which would be difficult in a home consumer-type electronic organ. On the other hand, if these eight notes are mixed into one audio channel and speaker system, a single amplitude modulated signal is radiated into the room and all the beats would be presented to both ears of the listener in the same way, with no differential effects, thereby depriving the listener of any spatial information.
For a better understanding of this phenomenon, consider two equal amplitude sinewaves, one having a frequency of 500 Hz and the other a frequency of 502 Hz. If these signals are electrically mixed, amplified and reproduced through a loudspeaker, a beat frequency that is alternately loud and soft two times per second will be heard; in a dead room the beat signal would be loud to both ears of a listener at the same time and soft to both onefourth second later. In order for a listener to perceive spatial information, it is necessary that the signal at one ear be increasing in loudness at the same time it is decreasing or staying constant at the other ear, and vice versa. This can be accomplished in a number of ways with varying degrees of effectiveness. In the simplest case, if the speaker in the above example is placed in a very "live" room having a reverberation decay time of approximately one second, and sound is radiated into the room from the speaker, standing waves are set up. In the case of a single sound source, standing waves result from the total reflected sound being of such phase and amplitude as to reinforce the direct sound or to reduce, or in some cases, cancel it. This means that a steady state tone will have a wide range of loudness levels throughout the room and in some places it will cancel out completely. The distance between these peaks and valleys will be determined by the frequency and thus the wave length of the sound signal. If we humans did not have two ears spaced apart, this would present real listening problems; however, being spaced apart, one ear can be positioned at a null while the other is located at a point of fairly high intensity, thus greatly reducing the apparent loudness variations one perceives as one moves about the listening room. At low frequencies the ears are too close together relative to the wave length of the sound signal and standing waves do sometimes give real listening problems. If now the intensity of the sound source in a reverberent room is changed fairly rapidly, the standing waves will all change their positions and continue to change their positions until the room sound level has stabilized at the new sound level of the source. This effect is more apparent when the loudness variations are caused by a beat between two frequencies than variations caused by simple amplitude modulation of a single frequency; this is so because a beat note is equivalent to a frequency half way between the two beating frequencies being combined in a balanced modulator with the frequency difference between them (i.e., the 501 Hz modulated by 2 Hz). Since a balanced modulator alternately reverses the phase of the modulated signal, the 501 Hz signal would build up in loudness, then go to zero and start building up 180.degree. out of phase with its prior value, then back to zero again followed by buildup to again be in phase with the original value. This phase reversal will cause the standing waves to momentarily change places with each other, because in those places where the direct sound was reinforcing the reflected sound it will for a short time cancel or reduce the level, and in those places where the direct sound was cancelling the reflected sound, it will now reinforce it. Thus, it will be seen that a continually changing loudness creates a dynamic situation which, in a live room, changes the standing waves, which is perceived as a spatial effect because it will not be identical at both ears of the listener at the same time. The effect is otherwise in a dead room because there will be no sound in the room to react one-half second later with the direct sound.
Consider now the case where there are two separate sound sources; for example, a 500 Hz sound signal radiated by one speaker and a 502 Hz sound signal radiated by a second speaker. At some point in time the cones of the two loudspeakers will be moving in phase with each other, and if it is assumed that the speakers are in a free space with no reflected sound, the sound in the area equidistant from the two speakers will be reinforced. If now the listener moves to one side so that one speaker is about one foot further from the listener than the other one, a certain amount of cancellation will occur because the wave length of a 500 Hz sinewave is a little over two feet; thus, in traveling one foot the phase will be reversed. Accordingly, interference patterns will be set up, even in free space. In a dead room with reflections there will again be standing waves, but the standing waves will be different for each speaker because of their different locations in the room and the fact that sound coming from both speakers contributes to a standing wave pattern in the room. Because of the frequency difference of the sound signals, a little time later the cones of the two loudspeakers will be moving in opposite directions, causing all the standing waves to move and create a dynamic effect. In this case, however, the effect does not depend on reverberation and a pleasant spatial effect can be perceived even in a dead room.
A similar result is achieved by the system disclosed in the aforementioned Leslie U.S. Pat. No. 3,049,040, wherein one of a set of tone generators is coupled to both channels of a pair in one fixed relative phase relationship and another generator of the set is coupled to both channels in a second different relative fixed phase relationship, each channel having a separate transducer physically separated from the other. If, for example, 500 Hz and 502 Hz signals are electrically combined in one of the channels, and the phase of the 502 Hz signal is reversed and electrically combined with the 500 Hz signal in the second channel, when the signals are in phase they will reinforce each other in one channel and will cancel each other in the other channel, and vice versa. This causes a dynamically changing standing wave pattern in the room which sounds approximately the same as the two-source arrangement discussed previously, but has the important advantage that it is not necessary to combine the signals 180.degree. out of phase for the system to work; for example, a phase shift of 90.degree. works quite satisfactorily. This makes it possible to combine the aforementioned eight channels in different phase relationships into two channels without destroying the spatial effects, and obtaining a result equivalent to what happens when a stereophonic recording is made. In stereophonic recording, because of the different distances the microphones are from each separate sound source, the phases of each of the signals as they arrive at the two microphones will be different and statistically most of the spatial information will be preserved. Although the apparatus disclosed in U.S. Pat. No. 3,049,040 theoretically is capable of generating a stereophonic effect, as a practical matter it is difficult and extremely costly, particularly in analog organ designs, to provide separate phase shift circuits for each signal for coupling them into two separate sound channels.
Another departure from realism caused by electrical combination of the outputs of two or more oscillators or tone generators of an electric organ is that the buildup of acoustic energy at the ears of the listener when a single note at different stops are sounded does not correspond to what happens in the real world and therefore lacks the pleasing effect of a pipe organ where sounds from two or more sources are acoustically combined. More specifically, when two electrical sound-representing signals each having an amplitude of one volt, for example, are electrically combined, the resultant signal has an amplitude of two volts and, since the acoustic power developed by a loudspeaker is proportional to the square of the voltage, the resulting acoustic power goes up by a factor of four, whereas if the two signals were reproduced separately and acoustically combined, the energy would only be doubled. These fundamental laws create severe problems in electronic organ design, such as the need to leave adequate amplifier head room to accommodate the signals. The problem was recognized by the developers of the digital organ described in U.S. Pat. No. 4,202,234, and was solved by performing, in the waveform compiler, the square root of the sum of the squares of the amplitudes of all harmonics; accordingly, instead of the addition of two equal amplitude signals resulting in a doubling of amplitude, they are added according to a square law function.
The primary object of this invention is to provide a simple arrangement of tone generators, electrical circuits and electrical-acoustic channels for generating a stereophonic sound image.
Another object of this invention is to provide apparatus for minimizing beat effects which, at the same time generates a stereophonic sound image.
Yet another object of this invention is to improve the realism of an electronic organ.